1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a thin film transistor (TFT) of a liquid crystal display device and a method of fabricating the same, which improves the on-current characteristic of the LCD device and reduces parasitic capacitance by adjusting a width of an electrode of the LCD device.
2. Discussion of the Related Art
Generally, an LCD device includes top and bottom glass plates, and a liquid crystal injected therebetween. On the bottom glass plate, a plurality of gate lines extending in one direction and a plurality of data lines extending in another direction perpendicular to the gate lines are formed. In this matrix arrangement, a plurality of TFTs and a plurality of pixel electrodes are disposed at the cross-over points of the data and gate lines.
On the top glass plate, red (R), green (G) and blue (B) color filter layers and a common electrode are disposed. Generally, a light shielding layer (black matrix) is formed on the top glass plate and a pair of polorizers are disposed on the outer surfaces of the top and bottom glass plates to selectively transmit light.
A conventional LCD device will be described below with reference to FIG. 1 which shows a layout of a conventional LCD device.
As illustrated in FIG. 1, the conventional LCD device includes a plurality of gate lines 1 formed on a transparent substrate, a plurality of data lines 2 crossing the gate lines 1, a plurality of TFTs 5 formed at the cross-over points of the gate and data lines 1 and 2, and a plurality of pixel electrodes 3 connected to the TFTs 5. The gate lines 1 are separated by certain intervals and extend in one direction, whereas the data lines 2 are separated by certain intervals and extend perpendicular to the gate lines 1. Two adjacent gate lines 1 and two adjacent data lines 2 crossing the gate lines 1, define the boundaries of a pixel region. In each pixel region, the TFT 5 and the pixel electrode 3 are disposed.
Each of the TFTs 5 includes a gate electrode 1a, a source electrode 2a, and a drain electrode 4. A gate insulating layer is formed between the gate and source electrodes 1a and 2a and between the gate and drain electrodes 1a and 4. The gate electrode 1a extends from the gate line 1 and the source electrode 2a extends from the data line 2. The drain electrode 4 connects the gate electrode 1a and the pixel electrode 3.
The TFT 5 transmits a signal of the data line 2 to the pixel electrode 3 in response to a signal of the gate line 1. The source electrode 2a and the drain electrode 4 have the same width (a).
In the conventional LCD device having the above-described TFTS, if a signal voltage is applied to the gate electrode 1a, the TFT is turned on so as to transmit a data voltage representing picture data to the pixel electrode 3 and the liquid crystal..
Given that a current required to charge the liquid crystal is known as the "on-current" , I.sub.ON, a general expression of I.sub.ON is as follows: ##EQU1## where .tau..sub.g is a gate turn-on time, C.sub.TOT is a total capacitance of the liquid crystal and an auxiliary capacitor, and V.sub.LC is a voltage applied to the liquid crystal.
On the other hand, there are cross-over portions where the gate electrode 1a overlaps with the source electrode 2a and the drain electrode 3. In this case, the cross-over portions have parasitic capacitance of C.sub.gs and C.sub.gd, where C.sub.gs is the gate-source capacitance and C.sub.gd is the gate-drain capacitance.
When the TFT is turned off, the voltage on the pixel drops as much as an amount .DELTA.V.sub.p due to capacitive coupling. The drop amount .DELTA.V.sub.p can be expressed as follows: ##EQU2## where C.sub.gs is the gate-source capacitance, C.sub.LC is the pixel capacitance, C.sub.st is the storage capacitance, and V.sub.g is the gate voltage. .DELTA.V.sub.p also adds to the deterioration in the picture quality of the LCD device. Therefore, in order to improve picture quality, it is important to minimize .DELTA.V.sub.p and improve on the on-current characteristics of the LCD device.
However, the conventional LCD device includes source and drain electrodes having the same width. Such a structure neither improves the on-current characteristic nor reduces parasitic capacitance. Therefore, the conventional LCD suffers from a deteriorating picture quality.